The present invention is concerned with a method and apparatus for drying a gas, such as air, with improved efficiency. More particularly, this invention relates to rotary desiccant systems.
In many industries, atmospheric moisture control is absolutely essential. For instance, materials in storage may be damaged by condensation of water from humid air. In addition, hygroscopic materials collect and hold moisture from the air, requiring strict humidity control procedures. For example, processes for manufacturing silicon chips for electronic circuits must be performed in a humidity controlled environment to protect the integrity of the chips.
One conventional method and apparatus for dehumidifying, or controlling the humidity of a gas, such as atmospheric air, employs a rotary sorption bed system. This system utilizes a desiccant, or an active sorbent, such as lithium chloride, or other adsorbent or absorbent materials. In the typical rotary bed dehumidifier, air that is to be dehumidified, known as process air, is passed over or through a rotating bed carrying the particular sorbent material. The sorbent material draws the humidity from the process air, and the resulting dry air is exhausted into the humidity controlled room. The sorbent material in the sorption bed passes from the process sector to the regeneration sector as the bed rotates, wherein the bed is regenerated. In the regeneration zone, regeneration air is passed over or through the desiccant material to extract the humidity from the sorbent material.
Typically the regeneration air is heated prior to entering the regeneration zone in order to increase its capacity to hold moisture and to remove moisture from the desiccant. Once the desiccant, or sorbent material, has been regenerated, it passes to the process sector as the bed rotates ready to absorb moisture from the process air. In the prior art, to control capacity the sorbent material is only partially regenerated prior to entering the process sector. This minimizes the temperature differential between the process and regeneration sectors.
In many applications of the rotary bed dehumidifier, it is necessary to cool the process air is cooled prior to being exhausted into the humidity controlled room due to the sensible heat that the process air picks up heat from the sorbent material and from the conversion of moisture content in the process air to sensible heat. In some prior art rotary bed devices, a cooling zone is added between the regeneration zone and the process zone of the rotary bed in which cool air is passed over or through the desiccant material to cool the material prior to entering the process zone.
Several attempts have been made in the prior art to improve the efficiency of the rotary bed dehumidifier. For instance, Norback, in U.S. Pat. No. 4,529,420, proposed recirculating the exhausted regeneration air, heating the recirculated regeneration air, and directing the recirculated air to the first of two regeneration zones of the rotary bed. Norback also suggests the use of a separate cooling airflow through a cooling zone interposed between the regeneration zone and the process zone of the rotary bed. In the patent to Oliker, U.S. Pat. No. 4,701,189, several recirculation paths are proposed in which the regeneration air is recirculated to increase its moisture content and, ultimately, the regeneration capacity of a given volume of air. In Macriss et al., U.S. Pat. No. 4,134,743, a heat exchanger is interposed between the dry product stream and the ambient regeneration airstream so that heat is withdrawn from the dry product and used to partially preheat the ambient regeneration air prior to entering the regeneration zone of the rotary bed.